1. Field of the Invention
The invention relates to the field of magnetic bubble memories, particularly those employing permalloy propagation elements.
2. Prior Art
Magnetic bubble memories employing a magnetic garnet layer in which magnetic bubbles are propagated and overlying permalloy propagation elements are well-known. Typically, the bubbles are propagated in parallel lines (in both directions) under the influence of a rotating magnetic field. At the ends of these lines, larger propagation elements are used to urge the bubbles from one line to another.
Much effort has occurred in optimizing the shape of these propagation elements. Dozens of different chevron-shaped elements are used or proposed for providing more efficient bubble propagation. See for instance, "Characteristics and Test Results for 272k Bubble Memory Package", IEEE Transactions on Magnetics, Vol. MAG-13, No. 5, September 1977 by Boebeck and Danylchuk; and "68k Bit Capacity 16 .mu.m-Period Magnetic Bubble Memory Chip Design with 2 .mu.m Minimum Features", IEEE Transactions on Magnetics, Vol. MAG-12, No. 6, November 1976 by Bonyhard and Smith. Also see U.S. Pat. Nos. 3,940,631; 3,940,751; 4,117,544; 4,151,601 and 4,276,613.
In general, a particular propagation element is selected for a magnetic bubble memory and this propagation element is used throughout the memory. The propagation elements are selected to provide optimum bubble transfer, generally with an eye to reducing the magnitude of the rotating magnetic field. Reduction in the strength of this field reduces power dissipation.
It apparently has been assumed that the propagation characteristics for a given permalloy propagation element are the same, or approximately the same in any direction. That is, the characteristics for a given chevron element are the same when the bubble is transferred in a first direction or in a second direction opposite to the first direction. As will be seen, the present invention departs from this assumption, accepts the asymmetry apparently inherent in the fabrication of magnetic bubble memories and provides a method for selecting propagation elements based on the asymmetry.